One can measure surface roughness either by using a contact or non-contact method. Some other optical microscope-based methods may be more qualitative.

Contact methods include using atomic force microscopy or surface profilometry. Essentially, run a stylus on the surface and track the rise and fall that corresponds with changes in the surface roughness. The measurement would give you the heights of the valleys and the depths of troughs. You can also calculate a root mean square roughness or RMS roughness. 

Examples of non-contact methods include interferometry that uses white light or discrete wavelengths. 

 The length of the scan essentially depends on the planarity of the sample surface. At some point the global curvature of the sample is going to start influencing the measurements.

I hope this helps.
Bhanu

> On Nov 13, 2015, at 7:52 AM, Eddie Hofer <[log in to unmask]> wrote:
> 
> Wondering if anyone has more experience in the measurement of surface
> roughness.  I am hoping to better understand the specifications
> relating to surface roughness and specifically the nuances behind
> determining sample length.
> 
> Appreciate any input you can provide.
> Thanks!
> Eddie Hofer
> 
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